Conventional processes for the recovery of high purity aromatic hydrocarbons such as benzene, toluene and xylenes (BTX) from various hydrocarbon feedstocks including catalytic reformate, hydrogenated pyrolysis gasoline, etc., utilize an aromatic selective solvent. Typically, in the practice of such processes, a hydrocarbon feed mixture is contacted in an extraction zone with an aqueous solvent composition which selectively dissolves the aromatic components from the hydrocarbon feedstock, thereby forming a raffinate phase comprising one or more non-aromatic hydrocarbons, and an extract phase comprising solvent having aromatic components dissolved therein.
An important consideration in the design and operation of an aromatic extraction process beyond the recovery and the purity of the products is the operating cost. Excluding labor and capital charges, the major component of the operating cost is the energy requirements of the process. Because the products produced from this process are generally not finished products, but are often intermediate components for the blending of finished fuels and for the production of petrochemicals, any reduction in processing costs can translate into a significant economic advantage. An example of the energy costs for a conventional process for this application appeared in a discussion of the UOP Sulfolane Process which may be found on pages 8-53 to 8-60 in a book edited by Robert A. Meyers, entitled Handbook of Petroleum Refining Processses, published by McGraw-Hill Book Company, in 1986. In Table 8.4-1 on page 8-60 of this reference, the processing costs are presented for a 10,000 (B/D) barrel per day Sulfolane Unit producing 6,000 B/D of BTX extract. The energy costs, comprising steam, electric power and cooling water amounted to 83% of the total processing cost on a daily basis, expressed in monitory units. Solvent make-up charges were 2%. Labor and maintenance costs made up the remaining 15%.
In commercial practice the amount of energy used by aromatics extraction processes range from 600-900 BTU's (British Thermal Units) per pound of aromatic material extracted. When one considers that the recovery of aromatic hydrocarbons involves the use of three major material cycles, typically hydrocarbon, water, and solvent, there are numerous possibilities within the process for the management of energy as these materials are vaporized and condensed to affect the separation.
A number of examples have been developed to achieve energy savings using continuous solvent extraction-steam-distillation for the recovery of aromatic hydrocarbons. Representative examples are believed to be presented in U.S. Pat. Nos. 4,690,733; 4,693,810 and 4,664,786. The U.S. Pat. No. 4,690,733, issued to Forte et al., shows an aromatics extraction process segment wherein high pressure steam is fed to a steam ejector and the resulting steam is employed in a first heat exchanger to provide heat to reboil a distillation column for the recovery of lean solvent and in a second heat exchanger to transfer heat from the recovered lean solvent to the condensate from the first heat exchanger before returning the condensate to the steam ejector to complete the steam generation cycle. The process is claimed to lower energy costs by reducing reflux to feed ratios in the extractor and lowering solvent recirculation rates. Capital costs are increased by the addition of the steam ejector and an additional heat exchange is required over conventional practice.
In the patent to Forte et al. (U.S. Pat. No. 4,693,810), the stripping water is divided into two streams. One stream is passed to a motive steam generator to vaporize the water and then passed to a steam ejector. The second stream is passed to a heat exchanger wherein heat is transferred from a lean solvent stream to vaporize all of the stripping water rather than just a portion and return the water vapor to the stripping column by means of the steam ejector. Capital costs are increased by the cost of the steam generator, steam ejector, and additional heat exchanger.
In the patent to Forte et al. (U.S. Pat. No. 4,664,786) stripping water is recycled to a steam distillation zone while heat recovered from an overhead stream is used to vaporize the stripping water using a motive steam generator to pump stripping water vapors into the stripping column.
Other processes for producing high purity aromatics are described in U.S. Pat. No. 3,714,033, and U.S. Ser. No. 408,827, allowed Jan. 16, 1991. These processes provide for the use of a single distillation column wherein both extractive distillation and a steam stripping occur. The patent discloses the preferred use of a polyalkylene glycol solvent in a temperature range of from about 100.degree. C. (212.degree. F.) to about 200.degree. C. (392.degree. F.) to provide a high purity aromatics product.
One more process for producing high purity aromatics is described in U.S. Pat. No. 4,058,454 and provides for the use of extractive and steam distillation in separate columns. A particularly suitable class of solvents for use in the above-identified patent are those commonly referred to as the sulfolane type. The process utilizes an extraction temperature, with a sulfolane solvent, in the range of from about 80.degree. to about 400.degree. F. and can provide a high purity aromatic product.
U.S. Pat. No. 3,590,092 discloses a method for aromatic hydrocarbon recovery that utilizes a single column wherein a side-cut vapor fraction comprising aromatic hydrocarbons and a minor quantity of solvent is withdrawn and introduced into a separate rectifying zone maintained under rectifying conditions, to provide a relatively solvent free aromatic extract product.
U.S. Pat. No. 3,702,295 discloses a method for aromatic hydrocarbon recovery that also utilizes the single column, vapor side-cut approach. However, this method differs from that disclosed in U.S. Pat. No. 3,590,092 in that the rectification zone is refluxed with the aqueous phase from the overhead condensate, instead of the hydrocarbon phase. Also, the bottoms fraction from the rectification zone is introduced to an intermediate section in the stripped column instead of the lower section.
Another aromatic hydrocarbon recovery method that uses a single column to provide two stripping sections is shown in U.S. Ser. No. 321,033, allowed Jan. 28, 1991. This arrangement uses a separate rectification column for purification of the aromatic product, but teaches that the rectification zone may be incorporated into the column.
Overall energy requirements of the process can be significantly reduced by minimizing the amount of heat withdrawn from the process to the environment as in cooling, or condensing of process streams. The condenser duty translates into the amount of energy that cannot be recovered, that is lost or taken out of the process. By lowering the temperature of the lean solvent, the selectivity of the solvent is improved and the amount of non-aromatics extracted with the aromatics is reduced, reducing the overall energy consumption.
Another way in which energy requirements can be reduced is by the utilization of less expensive heat sources. Many solvent extraction processes have arrangements that need high pressure steam to provide effective heating. Arrangements that permit the substitution of low pressure steam for heating--refiners generally view low pressure steam streams as waste heat--will offer significant savings over processes that need high pressure steam.